Try using below code:

url: `${this.props.siteurl}/_api/web/lists/getbytitle('Templates')/items?$select=Title,CurrentLink,HistoricalLink,DataGroupTemplate/Template&$expand=DataGroupTemplate&$filter=DataGroupTemplate/Template eq '${a}'`

Try using below code:

url: `${this.props.siteurl}/_api/web/lists/getbytitle('Templates')/items?$select=Title,CurrentLink,HistoricalLink,DataGroupTemplate/Template&$expand=DataGroupTemplate&$filter=DataGroupTemplate/Template eq '${a}'`

Update:

Instead of ${this.props.siteurl}, try using ${reactHandler.props.siteurl}.